In the world of synaesthesia, words can be tasty and tastes can have physical shapes – but neuroscientists aren’t quite sure why.

James Wannerton says his favourite people to listen to are Americans who speak quickly. “A well-elocuted UK accent I'd have difficulty listening to,” says the IT professional, “Americans tend to mingle and mangle. You get your 'd's and 't's mixed up, ‘medal’, ‘mettle’, all that stuff. It makes you very easy to listen to, because I don't get so many tastes and flavours to distract me.” He adds, perhaps as a nod to his extremely speedy American interviewer, “I mean, it's an advantage.”

For Wannerton, words are a constant source of distraction because the consonants give them taste. “College” tastes of sausage. “Karen” tastes of yoghurt. “Yoghurt” tastes, foully, of hairspray. “Most” tastes of “crisp, cold toast with hardly any butter on it”. “BBC Future” tastes of syrupy tinned peaches. And the names of stations on London’s transport network conjure up a cornucopia of weird and wonderful tastes. The experience can be overwhelming, akin to having a bright light shown in one's face while one tries to go about one's business. “Normally I would refrain from using the word ‘most’,” he relates. “I would never write it. I'd find an alternative. It distracts me, and all I can think of is toast all the time.” And he laughs, a little helplessly, at how he lives at the mercy of his unusual talent.

Sensory experiences are intensely personal and hard to convey to other people. What tastes amazing to one person may be nothing special to another, and you'd be hard-pressed to say exactly why. A smell can evoke disgust or delight, and scientists who study colour vision hear endless stories of spouses disagreeing on whether the couch is black or very dark blue. Everyone's a little bit different, for reasons of biology, experience, and personality. But some people, synaesthetes like Wannerton, are very different indeed. And they might not know for years and years.

Pointy’ chicken

Take, for instance, the case of Michael Watson. As neurologist Richard Cytowic remembers it in his book Wednesday is Indigo Blue, the moment of discovery came when Watson was hosting a dinner party in February 1980 where Cytowic was a guest. Watson, dismayed, told the diners he'd got something wrong: there weren't “enough points on the chicken”. Cytowic asked Watson to explain what he meant: “'With an intense flavour,' he explained, 'the feeling sweeps down my arm into my hand and I feel shape, weight, texture, and temperature as if I’m actually grasping something.' He had wanted the chicken to be a prickly, pointed sensation, 'like laying my hand on a bed of nails.' But it came out all round.” Cytowic was fascinated by this statement, which he credits with kicking off his pioneering work in the study of such automatic cross-sensory experiences, or synaesthesia.

Watson had different tactile sensations for all kinds of tastes – spearmint, for instance, gave him a feeling of running his hand along a tall, cool column of glass or marble – and while Cytowic found that most of his colleagues maintained Watson must be making up the experiences, for attention, Watson's sensations were remarkably consistent. They seemed to be as natural to him as a normal taste perception is to everyone else, which is why he hadn't discussed them before.

In the 35 years since that dinner party, the study of synaesthesia has exploded. We now know that synaesthesia is quite “real”, and there are all kinds, a menagerie of crossed sensations. Some synaesthetes see letters and numbers as coloured, known as colour-grapheme synaesthesia; some hear sounds when they read them; some perceive dates and months as existing in a three-dimensional space that they can scroll through.

Altered genome

A few have taste synaesthesias, like Wannerton and Watson. Bring the phenomenon up in the conversation at a dinner party, and you may find you have synaesthetes hidden among you. I had been with my husband for more than two years before the fact that vowels have colour for him ever made it to the head of the conversational queue, and it was a party discussion that did it.

Some forms may be inherited – synaesthesias can run in families, and in 2009 researchers fingered four regions on the genome that seemed be altered in people for whom sound triggers a sensation of colour. Neuroscientist David Eagleman, who co-wrote Wednesday is Indigo Blue with Cytowic, has identified a chromosomal region that's linked to synaesthesia where a letter, number, day, or month provokes a colour. And when synaesthetes have their brains scanned while they read, speak, or do whatever it is that activates their unusual perceptions, blood flows into areas of the brain that aren't active in people with standard sensations.

These are areas implicated in taste, touch, or whatever other axis is involved. But the connection between the genetic correlations and what comes up on brain scans – in other words, how exactly the synaesthetic experience is produced biologically, beyond the broad statement that other senses are invoked – is still mysterious.

Early childhood experiences seem to play into the specifics of some people's synaesthesia. For instance, scientists studying Wannerton had his parents fill out a questionnaire on what they'd fed him growing up, and many of his tastes showed up on it. But, curiously, not all his tastes correspond to food he's had before. For a long time the salty, savoury taste of “except” was familiar, but it had a crunch to it that didn't seem to belong, he says. Then he happened to try some Marmite crisps. He'd always enjoyed Marmite spread, but the crisps were new to him, and they corresponded perfectly to the experience of the word.

Taste of writing

For his part, Wannerton first brought up his strange ability to his parents when he was about 10, he says, because the distraction of constant flavours washing over him made it difficult to read and study for school. Reading is still tough, but writing has always held a special kind of pleasure for him, as it allows him to assemble something that tastes lovely.

Once, when he was working as a reporter, he spent all night on a 900-word sports story about Northern Irish footballer George Best, choosing the words so that the introduction consisted of hors d'oeuvres tastes, the middle of main courses like roast beef, and the conclusion of dessert. “It was really good fun,” he says. “But the thing that put me off journalism was that the subeditors would swap words.”

He concedes that this symphony of taste can sometimes be overwhelming. But he can say something not all of us can: “I get tremendous joy out of writing the blandest email.”

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